Nasal cycle

A CT scan showing evidence of the nasal cycle. The more patent airway is on the right of the image; the swollen turbinates congesting the left.

The nasal cycle is the often unnoticeable[1][2] alternating partial congestion and decongestion of the nasal cavities in humans and other animals. It is a physiological congestion of the nasal concha due to selective activation of one half of the autonomic nervous system by the hypothalamus. It should not be confused with pathological nasal congestion. Although various aspects of the nasal cycle have been studied and discussed in the ancient Yogic literature,[3] in the modern western literature, it was first described by the German physician Richard Kayser in 1895.[4]

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In 1927 Heetderks[5] spoke about the alternating turgescence of the inferior turbinates in 80% of a normal population. The cycle is the result of alternating congestion and decongestion of the turbinates, predominantly the inferior turbinates, which are by far the largest of the turbinates in each nasal fossa. Turbinates consist of bony projections covered by erectile tissue, much like the tissues of the penis and clitoris. The turbinates in one fossa filled up with blood while the opposite turbinates decongested by shunting blood away. This cycle, which is controlled by the autonomic nervous system as described above, had a mean duration of two and a half hours. He further observed and documented that the turbinates in the dependent nasal fossa filled when the patient was in the lateral decubitus (lying down on your side) position. Some[who?] postulate that this alternating positional obstruction has the purpose of causing a person to turn from one side to the other while sleeping. Others note that the asymmetric airflow may have some benefit to overall olfactory sensitivity.[6] The nasal cycle is an alternating one, with the total resistance in the nose remaining constant. In patients with a fixed septal deviation and intermittent nasal obstruction, the interplay of the nasal cycle becomes evident; the sensation of obstruction frequently mirrors the congestion phase.[6]

It is possible that the Nasal cycle may exacerbate the nasal congestion caused by the common cold, as the lack of motility of the cilia in one half of the nose may lead to an uncomfortable sensation of not being able to shift mucus by blowing the nose.

It has been shown that the cilia of the congested side suspend their motility until that side decongests. Thus the cycle ensures that one side of the nose is always moist, to facilitate humidification, which is one of the three functions of the nose, the other two being filtration and warming of inspired air prior to its entering the lungs. [6]

Some odor chemicals bind with olfactory receptors easily, even under conditions of high airflow, and other odors need more time, under low airflow conditions, to bind with receptors. With high airflow on one side and low airflow on the other side, the olfactory center detects a greater range of smells.[7][8][9]

The nasal cycle should not be confused with pathological nasal congestion: individuals with normal nasal breathing usually do not realize their breathing is asymmetric unless there is underlying nasal obstruction.[10] In pathological conditions, however, the nasal cycle may influence the symptoms.[2]

In 1994, breathing through alternate nostrils showed effects on brain hemisphere symmetry on EEG topography.[11] D.S. Shannahoff-Khalsa published in 2007 on the effect of this cycle and manipulation through forced nostril breathing on one side on the endogenous ultradian rhythms of the autonomic and central nervous system.[12]